1. Field of the Invention
This invention relates to an optical head for optical recording, playback or erasure of signals representing data on a recording medium. In particular, it relates to an optical head wherein errors caused by differences in wavelength of the light beam employed are corrected.
2. Description of the Related Art
In a prior art optical head, a light beam having an elliptical cross-section is generated from a semiconductor laser. This light beam is then collimated in a parallel beam by a collimator lens and is incident on an inclined surface of a prism so that a correction for ellipticity of the beam is performed utilizing the refractive index of the prism. In this optical system, since the prism is made of glass having a chromatic dispersion, if the wavelength of the light beam is varied or altered, the light beam is directed in a undesired direction in the prism. As a result, when the light beam is converged on the optical disk to form a beam spot, this beam spot can not formed at a proper and displaced from the proper position. If such displacement is large, it resulted in a focusing error or tracking error. Furthermore, it resulted in a reading error due to a differences in retrieved signal.
Recently, a multi mode semiconductor laser which is oscillated in a stable manner has come to be employed. However, such semiconductor laser is operated in multiple modes so that it emits a laser beam having a number of wavelengths. As a result, the laser beam cannot be acculetly focused on the optical disk so that a deformed spot having a relatively large width elongated in a direction is formed.
There has been proposed a conventional optical system which cancels the influence of chromatic dispersion. In the conventional optical head, a pair of anamorphic prisms are arranged between a collimator and a polarization beam splitter. The laser beam generated from a semiconductor laser is collimated into a parallel beam by collimator lens, and the elliptical shape of the beam is converted into a circular shape by the anamorphic prisms. This circular beam is then directed to object lens through polarization beam splitter and a quarterwave plate.
The laser beam is converged by object lens and reflected from a recording surface of optical disk. The reflected laser beam is returned to the object lens and is guided to the polarization beam splitter through the quarter-wave plate. The laser beam is reflected by polarization beam splitter and is converged on a photodetector having four divided photosensitive sections by a converging lens and a cylindrical lens. Astigmatism is applied to the laser beam by the cylindrical lens so that the shape of the beam spot on the photodetector is changed depending on the distance between the optical disk surface and the objective lens. Thus, a focusing error of the objective lens is detected by the photodetector and objective lens is moved along its optical axis in response to electrical signals from the photodetector in such a manner that the shape of the beam spot becomes circular on the photodetector.
In an another conventional optical head, instead of the anamorphic prism, there are provided modified prisms joined each other to form a signal optical unit wherein the laser beam is emerged from the exit surface of the prism at a fixed angle with the normal to the exit surface.
In the conventional optical head described above, so-called chromatic correction is performed by addition of the anamorphic prisms. Thus, a relatively large is required number of components and size of the optical head can not be decreased. There is also required that the optical axes of these relatively larger number of components are aligned. Accordingly, in the conventional optical head, there is a problem that its cost can not be improved.